Fluid sampling device

ABSTRACT

A device for the collection, storage or transfer of fluids comprising two thin, metallic inherently stable shells each of like shallow dome-shape contour joined at their basal abutting edges to define in internal cavity isolated from the atmosphere, at least one of said shells having an inherent spring-like flexibility such that under the application of an externally applied force it is displaceable without rupture from the cavity defining relation of maximum volume to a second stable position of an inverse uniformly dome-shaped contour with the other of said shells in cavity defining relation of minimum volume, and reversely, and passage means integral with at least one of said shells for establishing fluid communication with said internal cavity.

[ 51 May 16, 1972 [54] FLUID SAMPLING DEVICE [72] Inventors: Louis A. Pogorski; Ernest M. Reimer, both of Toronto, Ontario, Canada Louis August Pogorski, Toronto, Ontario, Canada [22] Filed: Apr. 6, 1970 [2|] Appl.No.: 25,736

[73] Assignee:

521 u.s.c| ..222/2l1,222/2l4,222/464,

73/425.6 51 Int. Cl ..B65d37/00 [58] FieIdofSeaI-ch ..222/206,2l1,215,214,464,

1,948,306 2/1934 Meurer 1,751,341 3/1930 Leisse.; 2,982,286 5/1961 Welch, Jr 141/3 1 3 Primary Examiner-Samuel F. Coleman Assistant Examiner-James M. Slattery Attorney-Weldon F. Green ABSTRACT A device for the collection, storage or transfer of fluids comprising two thin, metallic inherently stable shells each of like shallow dome-shape contour joined at their basal abutting edges to define in internal cavity isolated from the atmosphere, at least one of said shells having an inherent springlike flexibility such that under the application of an externally applied force it is displaceable without rupture from the cavity defining relation of maximum volume to a second stable position of an inverse uniformly dome-shaped contour with the other of said shells in cavity defining relation of minimum volume, and reversely, and passage means integral with at least one of said shells for establishing fluid communication with said internal cavity.

12 Claims, 5 Drawing Figures Patented May 16, 1972 l VENTOR. LOUIS A. POGORSKI 8 ERNEST M. REIMER FLUID SAMPLING DEVICE This invention relates to a novel device useful for the sam pling or collection of a fluid, the storage or shipment of the collected fluid sample, or the transfer of the fluid sample all in an efficient, safe, leak-free and substantially contaminationfree manner.

More particularly this invention relates to a novel device for taking a measured sample of gas or liquid in a quantity sufficient for gas chromatographic, mass spectrometric or other applicable types of analyses, which device can be hermetically sealed to isolate contained fluid samples for storage or shipment or used for the transfer of the measured sample of gas or liquid to suitable testing apparatus or into any system by utilizing simple techniques dictated by the structure of the device itself, all in an efiicient, safe, leak-free and substantially contamination-free manner.

Rapid growth of technology in the fields, amongst others, of chemical process industries, earth sciences, environmental studies, and medical sciences, has created a pressing demand for efficient, accurate and economic sampling and analyses of fluids from a variety of sources. In many projects not only are the major components of the fluid to be identified and a quantitative analysis reported, but also it may be necessary to establish the concentration level of trace components. Other demands may call for safe, contamination-free and leak-free storage of a collected sample or the collection and transfer of valuable or dangerous fluids for the purpose of further processing or for direct application of such fluids to a system.

The amount of fluid required for the purposes outlined is usually rather small. In general less than a few ccs (NTP) of a fluid sample are required for gas chromatographic analysis. An even smaller sample is adequate for mass spectrometric and other types of instrumental analysis now available. Fluid quantities of the same order are suitable for injection of reactants or labelling compounds into process equipment, or for injection into human or animal tissues.

Analysis of a fluid usually can be carried out with satisfactory precision once the fluid sample has been transferred to the analytical apparatus. The reliability of the analytical results, however, depends not only on the precision of the measurement obtained with the analytical apparatus, but on the quality and quantity of the sample collected, preserved, and transferred. Unless the collection of a sample, its storage or transfer into the inlet system of analytical equipment are carried out in a substantially contamination-free, leak-free, deteriorationfree and in a reproducible manner, the results will be unreliable or of little or no value.

Representative contaminationand deterioration-free collection, preservation and transfer of fluid samples into the analytical systems still present serious technical and economic problems. This is particularly true if volatile samples are involved and trace analysis is desired. In many cases, samples cannot be fed directly into the analytical apparatus because the technical and/or economic considerations make location of the necessary apparatus at the sample source, or placing of the sample source close to the location of the apparatus, impossible or impractical. In such cases, collection of the samples in suitable containers for-transfer to a laboratory for analysis represents the only practical way of approach. Many of the chemical process samples, environmental, geochemical and medical samples fall into the above category.

Known devices used for collection or storage or transfer of fluid samples for the purposes outlined, prior to this invention include the following:

a. glass collecting tubes or containers equipped with one or two stopcocks;

b. metal cylinders or tubes equipped with one or two valves;

c. glass containers equipped with breakseals;

d. piston-equipped syringes made of glass, metal, plastic or a combination of such materials;

e. bellows made of plastics, rubber or metal.

In order to collect a representative sample, the known devices of fixed volume must be properly purged or evacuated prior to the sample admission.

Purging of a container is a tedious operation. Fluid pockets in or around the valves, stopcocks or container ends are difficult to remove and tend to contaminate the contents. Large volumes of fluid to be sampled are used up for the purging operation. This method is, obviously, not suitable when the amount of the fluid to be sampled is small.

Evacuation of the known devices presents difficulties. Onlocation evacuation requires relatively complex equipment and is time consuming. Pre-evacuation may be only partially effective on devices employing valves and stopcocks. The preservation of a vacuum or of a stored fluid for an indefinite period of time is difficult to achieve, if not impossible.

Transfer, and in particular quantitative transfer, of a sample kept at the given charge pressure inside a storage container into the inlet system of the analytical instruments or into other systems operating, in general, at the pressures different to that of the given pressure of the sample presents still other disadvantages. Auxiliary equipment and complex time-consuming procedures subject to many errors are required to effect a satisfactory transfer.

The syringes or bellows-type devices overcome some of the disadvantages while posing others. The main disadvantages of the syringe-type devices are that they leak and that the seal on the piston presents adsorption and contamination problems. A permanent, herrnetical seal at the piston is not possible. Such devices cannot be used for storage of the samples for indefinite periods.

The main disadvantage of the bellows-type devices is the relatively large dead volume of the convolutions. The purge requirements and the poor reproducibility of the sample transfer operation caused by the dead volume and by a rather limited capability to collapse make this type of device unsatisfactory for many applications.

It is therefore the principal object of this invention to provide a device which greatly simplifies the task of collecting and transferring fluid samples for testing purposes or otherwise, and that may be used, if desired, for storing a collected fluid sample for an indefinite period of time without leakage, all in a substantially contamination-free manner.

More particularly, it is the principal object of this invention to provide a device that may be used for collecting a fluid sample from a source at atmospheric, sub-atmospheric or above atmospheric pressures, for storing the collected fluid sample for an indefinite period of time without leakage and deterioration and from which the collected fluid sample can be discharged into a system at atmospheric, sub-atmospheric or above atmospheric pressures, all in an efficient, safe and substantially contamination-free manner.

Still another important object of this invention is to provide a device of the character described, which is inherently adapted for collecting, storing and transferring a fluid sample in a quantitatively accurate and reproducible manner.

Still other important objects of this invention are to provide a device of the size that may be readily employed as a hand instrument, simply yet strongly constructed so as to be capable of easily withstanding the normal hazards of handling or shipping and which can be economically manufactured.

The principal feature of this invention resides in providing a device in which the walls of the fluid container portion are so shaped and at least one wall possesses such inherent resiliency that under the application of directed externally applied forces the inherently resilient wall is displaceable without rupture whereby the configuration of the container portion can be selectively converted from one inherently stable configuration enclosing a maximum specified volume to a second inherently stable configuration enclosing a minimum specified volume, and reversely, the vacuum created by the conversion of the container portion from its stable configuration enclosing the minimum specified volume to the configuration enclosing the maximum specified volume serving as the driving force to draw in a fluid sample and the reduction in volume by conversion of the container portion from its configuration enclosing a maximum specified volume to its configuration enclosing the minimum specified volume serving to expel the fluid sample therefrom.

More particularly, it is a feature of this invention to provide a device of the character described in which the container portion can be substantially completely collapsed, thereby reducing the minimum specified volume to a negligible quantity.

Still another important feature resides in providing a device of the character described in which the dimensions of the fluid container and wall thicknesses have been selected such that the container can be converted from one stable position to the other and reversely under finger and thumb pressure, and thereby serve as a hand instrument.

Still another feature of the invention resides in providing a device of the character described, in which the fluid inlet-outlet passage formation is in the form of a hollow needle or capillary tube, which may be suitably sharpened at its external tip for insertion into the septum of a system from which a fluid sample is to be drawn or into the septum of the inlet system of analytical apparatus into which the fluid sample is to be injected. Further the capillary tube may be so conditioned if necessary, throughout a portion of its length that it is susceptible of deformation by crushing to seal the contents of the container for storage or for shipment and then severed and sharpened if necessary, when it is desired to expel the contents.

These and other objects and features will become apparent in the following description to be read in conjunction with the accompanying sheet of drawings wherein FIG. 1 is perspective view on a slightly enlarged scale of the preferred embodiment of a device made in accordance with the invention showing the container portion in its stable configuration enclosing the maximum specified volume;

FIG. 2 is an enlarged vertical sectional view of the device of FIG. 1 taken along the line 2-2 ofFIG. 1;

FIG. 3 is an enlarged vertical sectional view of the device of FIG. 1 with the container portion thereof shown in one stable configuration of its collapsed state;

FIG. 4 is an enlarged vertical sectional view of the device of FIG. 1 with the container portion thereof shown in the other stable configuration of its collapsed state;

FIG. 5 is a perspective view of an alternative embodiment on a slightly enlarged scale made in accordance with the invention with the container portion in its stable configuration enclosing the maximum specified volume, partly broken away.

In the preferred embodiment illustrated in FIGS. 1 to 4 it will be observed that the device made in accordance with the invention comprises a capsular container portion provided with a fluid inlet-outlet in the form of a capillary tube 12.

The walls 14 and 16 of container portion 10 in the preferred embodiment are identical. Each has a peripherally flattened basal edge'portion l8 and 20 respectively and a shallow domeshaped configuration 22 and 24 respectively. Each wall 14 and 16 may be formed in a suitable die from a sheet of suitable metal, or metal alloy, preferably a sheet of stainless steel, the properties of which are particularly suited to the conditions normally encountered in the use of the device and possessed of characteristics essential to the successful operation of the device.

The abutting edges of walls 14 and 16 are edge welded together as at 26 throughout their peripheries adjacent their basal edge portions 18 and 20 to provide a strong hermetic seal in accordance with known welding procedures.

In the embodiment illustrated in FIGS. 1 to 4 the capillary tube 12, preferably made from stainless steel, is provided with a sharpened external tip 28 with the opposite end 30 disposed within a channel defined by depressions 32, 34 in the basal edge portions 18 and 20 of the walls 14 and 16. Capillary tube 12 is anchored within the channel by welding or in any other suitable manner to establish a strong hermetic seal.

In the preferred embodiment illustrated the peripheral outline of each wall 14 and 16 is circular. Further the domeshaped contour of each wall is quite shallow and preferably symmetrical about a common central axis 38.

In FIG. 2 the separation of the dome-shaped walls 14 and 16 in the stable configuration of container portion 10 enclosing the maximum specified volume is designated H and the diameter of the container portion 10 is designated D.

10 of maximum specified volume It has been demonstrated that by forming each wall 14 and 16 from a sheet of stainless steel having the following specifications, SS 301 spring temper 0.010 inch, and by selecting a diameter D of the order of 2 inches and by selecting a shallow dome-shaped contour such that the separation H is of the order of 0.2 a container portion 10 of maximum specified volume of the order of 2.5 c.c. can be provided sufficient for the sampling of fiuids for gas chromatographic analysis or mass spectrometric and other types of instrumental analyses and for various other uses.

Further it has been established that where the value of D/H is greater than 4 the container portion 10 can be converted from the apparent inherently stable configuration shown in FIG. 2 to that shown in FIG. 3 upon the application of external forces to the outer surface of wall 14 in the direction of the arrows 40.

Likewise container portion 10 can be converted from the apparently inherently stable configuration shown in FIG. 2 to that shown in FIG. 4 upon the application of external forces to the outer surface of wall 16 in the direction of arrows 42.

According to our observation under the application of the external forces in the direction of the arrows 40 to the wall 14 to collapse container portion 10, the wall 14 appears to pass through an over center position whereupon it snaps into a substantially inverse uniformly dome-shaped contour abutting the surface of the opposed wall 16 as illustrated.

Likewise upon the application of external forces in the direction of arrows 42 to collapse container portion 10 wall 16 appears to pass through an over center position whereupon it snaps into a substantially inverse uniformly dome-shaped contour abutting the surface of the opposed wall 14..

The application of external forces to the collapsed container portion 10 at the edges of the container portion 10 and in the direction of the arrows 44 shown in FIG. 3, or in the direction of the arrows 46 shown in FIG. 4, the walls 14 or 16 as the case may be appear to pass back through the over center position whereby they snap" back to their original dome-shaped contour thereby establishing the configuration illustrated in FIG. 2 enclosing the maximum specified volume.

It has been observed that not only is the configuration of container portion 10 illustrated in FIG. 2 apparently inherently stable but that the configurations of the collapsed container portion 10 illustrated in FIG. 3 and 4, enclosing the minimum specified volume, are also apparently inherently stable.

We have observed that a device of the character described may be collapsed and expanded without rupture. Moreover, we have also demonstrated that by exercising close control over the die forming operations and the welding procedures that the container portion 10 canbe converted from the configuration of FIG. 2 into the substantially completely collapsed configurations of FIG. 3 and 4 thereby minimizing the dead volume of the container portion 10.

Having outlined the structure and its utility in the collection, ples will now be described.

If it is desired to take a small sample of fluid for example 1 10 cc, from a system operating at atmospheric pressure, provided with a septum, first a device having a container portion of the order of I l0-cc is selected. Container portion 10 is then converted from the configuration shown in FIG. 2 to the collapsed configuration of either FIGS. 3 or 4 by the application of thumb and finger pressure which under ordinary conditions will be sufficient to exert the external forces necessary or if preferred by a pressure exerted by a suitable tool such as pliers, notched wrench or the like. The sharpened end 28 of capillary tube 12 may then be inserted through the septum of the system to place it in communication with the fluid source. By the application of thumb and finger pressure to exert the external forces necessary the collapsed container portion 10 is converted into the expanded configuration of FIG. 2 enclosing the maximum specified volume, the walls 14 and 16, upon separating, create a vacuum within the enclosed space which serves as the drivoperation of the device, storage and discharge of fluid saming force to draw the sample through the bore of the capillary tube 12 and into the enclosed evacuated space.

The capillary tube 12 can then be withdrawn from the septum of the system from which the sample has been taken and then inserted through the septum of testing apparatus if it is close by. By the application of pressure to convert the configuration of FIG. 2 to the collapsed configuration of either FIGS. 3 or 4 the fluid sample will be expelled into the system of the testing apparatus.

We have observed that with a device of the character described the reproducibility of sample collection and discharge is within 0.1 percent of container volume. Hence, where the apparent maximum specified volume of a given device has been established one may use such device for the collection, storage and discharge of a fluid sample of known volume, thereby eliminating any other procedure that would be otherwise necessary to measure the volume of the fluid sample collected and stored or transferred.

A device of the character described, slightly modified, can also be used to collect samples from a system operating at subatmospheric or at above atmospheric pressure and transferred to a system at atmospheric, sub-atmospheric or above atmospheric pressure in a substantially contamination-free and leak-free manner as will now be described.

The capillary tube 12 may be conditioned, as by annealing, or in any other suitable manner, such that is deformable throughout a portion of its length 48 remote from the sharpened tip 28, or close to the tip as preferred.

In the case where a sample is to be taken from a system operating at sub-atmospheric or at above atmospheric pressure the container portion is first converted from the expanded configuration of FIG. 2 to the collapsed configuration of either FIG. 3 or 4, and then the capillary tube 12 is inserted through the septum of the system from which the sample is to be taken. Upon conversion of the container portion 12 from the collapsed configuration of either FIGS. 3 or 4 to the expanded configuration of FIG. 2 a sample at sub-atmospheric or at above atmospheric pressure will be drawn into the evacuated space. Before the capillary tube 12 is withdrawn from the septum the deformable portion 48 of the capillary tube 12 can be crimped with a suitable instrument, such as a pair of pliers, thereby hermetically sealing the contents. The crimped capillary tube 12 can then be withdrawn from the septum of the system.

The samples collected and sealed in the manner described can be preserved indefinitely without leaks, contamination or deterioration, and the sealed devices may either be placed in storage or may be shipped to the laboratory in which tests are to be carried out.

When desired, the crimped portion of the capillary tube 12 can be ground off on a grinding wheel or severed by suitable shears, immediately prior to its insertion through the septum of the inlet system of the designated analytical apparatus.

In the alternative, procedures may be adopted whereby the crimped capillary tube can be severed after insertion through the septum of the inlet system of a designated analytical apparatus.

Container portion 10 may then be converted from the expanded configuration of FIG. 2 to the collapsed configuration of either FIGS. 3 or 4 to expel a metered quantity of the fluid sample collected.

It will be readily appreciated that where it is desired to hermetically seal a sample collected from a system operating at atmospheric pressure a similarly modified device may be employed.

It is contemplated that where, for example, a sample is to be drawn into the container portion 10 from a source at sub-atmospheric pressure auxiliary grips in the form of lever arms may be welded at the apices of the dome-shaped walls 14 and 16 to assist in the snapping of the walls 14 and 16 into the expanded configuration of FIG. 2 and to maintain them in such configuration, if necessary.

It is also contemplated that where, for example a fluid sample at substantially above atmospheric pressure is collected it can be expelled, if necessary, by the application of a mechanb cal force exerted by the shaped jaws of a vice.

By selecting a capillary tube 12 of a very small internal cross-sectional area through which fluid exchange may take place contamination of a collected sample may be minimized.

We have demonstrated that in a given system comprising container portion 10 and capillary tube 12, that if the total internal dead volume of the container system in the collapsed configuration is limited to the value shown in relation to the total internal volume of the said system in the expanded configuration according to the Table I, contamination of the collected fluid sample is minimized.

TABLE I Total internal dead volume of the collapsed container system Total internal volume of the expanded container system less THAN 0.05%

We have observed that under the conditions specified there is ample time to either grind the end of the capillary tube 12 to a needle point or for attaching a needle-type adaptor before inserting it into the septum of the analytical system, without appreciably affecting the results.

It will be obvious that container portion 10 can be provided with more than one capillary tube 12 for the purpose of introducing another fluid into the container portion or to change its composition, or to effect a reaction or to withdraw a portion of the fluid sample prior to its injection into the inlet system of analytical apparatus, or otherwise.

Capillary tubes attached to container portion 10 in accordance with this invention can be, if preferred, equipped with a pressure fitting, or threaded fittings of any type suitable for attaching them to the fluid source, to the inlet of analytical apparatus or to any other type of apparatus from which the fluid sample is to be drawn or into which the sample is to be discharged.

The fluid container portion 10 can be made to withstand pressures of the order of psig and is therefore suitable to serve under a variety of conditions.

In the modified embodiment of the invention illustrated in FIG. 5 the container portion 50 defined by shallow domeshaped walls 52 and 54 are adapted to be welded together throughout their entire peripheries. In this embodiment the capillary tube inlet-outlet passage is replaced by an aperture 56 of very small diameter which may be sealed by a drop of solder or by applying an adhesive metal tape. This modified instrument is applicable in circumstances where gas samples are to be collected from the atmosphere or from an open source at atmospheric pressure, for example in air pollution studies.

The fluid sample may be collected first by collapsing and expanding container portion 50 to flush out any residual fluid whereupon the container portion 50 can be finally expanded and aperture 56 sealed with a drop of solder or by the application of an adhesive metal tape.

The collected sample may be injected into the inlet of analytical apparatus against a suitably formed inlet system equipped with a piercing device whereby fluid communication is established in a substantially contamination-free manner and delivery of the contained sample accomplished upon collapsing the container portion 50.

The fluid inlet-outlet passage defined by aperture 56 may if preferred be provided with a hollow needle or capillary tube such as illustrated at 12 in FIGS. 1 to 4 which capillary tube can be suitably anchored therein or at any other location in the walls 52, 54 of container portion 50, as may be desired.

The embodiment of FIG. can be provided with more than one aperture or more than one capillary tube for the purpose of introducing a fluid to change the composition of a collected sample, or to effect a reaction, or to withdraw a portion of the fluid sample prior to its injection into the inlet system of analytical apparatus.

While stainless steel is the material from which the preferred embodiments have been made other suitable sheet material may be selected for particular applications. Also it should be mentioned that if the fluid sample to be collected is corrosive the surfaces in contact with the fluid can be plated or coated with a substantially non-corrosive, non-contaminating layer as the particular circumstances may dictate.

It will be understood that the preferred embodiments of the invention have been described and illustrated herein. Changes or modifications may be adopted by persons skilled in this field without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

l. A device for the collection, storage or transfer of fluids comprising two thin, inherently stable walls each of like shallow dome-shaped contour joined at their basal abutting edges to define an internal cavity of maximum specified volume isolated from the atmosphere, each of said walls having an inherent spring-like flexibility such that under the application of externally applied forces each wall is displaceable without rupture from a first stable position with the other of said walls in cavity defining relation of maximum specified volume through an over center position to a second stable position of an inverse uniformly dome-shaped contour with the other of said walls in cavity defining relation of minimum specified volume, and reversely, and passage means integral with at least one of said walls for establishing fluid communication with said internal cavity.

2. A device according to claim 1 in which said walls in said stable positions in cavity defining relation of minimum specified volume abut each other substantially throughout their areas.

3. A device according to claim 2 in which the basal abutting edges of said dome-shaped walls are circular.

4. A device according to claim 3 in which the value of the basal diameter to the maximum separation of the apices of said dome-shaped walls is greater than 4.

5. A device according to claim 4 in which said dome-shaped walls have a common axis of symmetry.

6. A device according to claim 4 in which said dome-shaped walls are each comprised of a thin sheet of stainless steel.

7. A device according to claim 6 in which the thickness of each sheet of stainless steel is less than 1 times the diameter of said dome-shaped walls.

8. A device according to claim 1 in which said passage means for establishing fluid communication with said internal cavity comprises a capillary tube extending through at least one of said dome-shaped walls and having a portion thereof extending outwardly beyond said dome-shaped wall.

9. A device according to claim 1 in which said passage means for establishing fluid communication with said internal cavity comprises a capillary tube arranged to extend between the basal abutting edges of said dome-shaped walls and having a portion thereof extending outwardly beyond said edges.

10. A device according to claim 8 in which said capillary tube is deformable throughout a portion of its length whereby said internal cavity can be hermetically sealed from the atmosphere.

11. A device according to claim 1 in which said passage means for establishing fluid communication with said internal cavity is defined by an aperture extending through at least one of said dome-shaped walls.

12. A device according to claim 1 in which the total minimum specified volume of said internal cavity and said passage means is of the order of 0.5 percent of the total maximum specified volume of said internal cavity and passage means, or less. 

1. A device for the collection, storage or transfer of fluids comprising two thin, inherently stable walls each of like shallow dome-shaped contour joined at their basal abutting edges to define an internal cavity of maximum specified Volume isolated from the atmosphere, each of said walls having an inherent spring-like flexibility such that under the application of externally applied forces each wall is displaceable without rupture from a first stable position with the other of said walls in cavity defining relation of maximum specified volume through an over center position to a second stable position of an inverse uniformly dome-shaped contour with the other of said walls in cavity defining relation of minimum specified volume, and reversely, and passage means integral with at least one of said walls for establishing fluid communication with said internal cavity.
 2. A device according to claim 1 in which said walls in said stable positions in cavity defining relation of minimum specified volume abut each other substantially throughout their areas.
 3. A device according to claim 2 in which the basal abutting edges of said dome-shaped walls are circular.
 4. A device according to claim 3 in which the value of the basal diameter to the maximum separation of the apices of said dome-shaped walls is greater than
 4. 5. A device according to claim 4 in which said dome-shaped walls have a common axis of symmetry.
 6. A device according to claim 4 in which said dome-shaped walls are each comprised of a thin sheet of stainless steel.
 7. A device according to claim 6 in which the thickness of each sheet of stainless steel is less than 1 times the diameter of said dome-shaped walls.
 8. A device according to claim 1 in which said passage means for establishing fluid communication with said internal cavity comprises a capillary tube extending through at least one of said dome-shaped walls and having a portion thereof extending outwardly beyond said dome-shaped wall.
 9. A device according to claim 1 in which said passage means for establishing fluid communication with said internal cavity comprises a capillary tube arranged to extend between the basal abutting edges of said dome-shaped walls and having a portion thereof extending outwardly beyond said edges.
 10. A device according to claim 8 in which said capillary tube is deformable throughout a portion of its length whereby said internal cavity can be hermetically sealed from the atmosphere.
 11. A device according to claim 1 in which said passage means for establishing fluid communication with said internal cavity is defined by an aperture extending through at least one of said dome-shaped walls.
 12. A device according to claim 1 in which the total minimum specified volume of said internal cavity and said passage means is of the order of 0.5 percent of the total maximum specified volume of said internal cavity and passage means, or less. 